This invention relates to systems and processes designed to treat monoethylene glycol (MEG) used in the oil and gas industry, especially in offshore locations, to control the formation of hydrates. More particularly, the invention relates to systems and processes that are designed to remove divalent ions from a MEG feed stream.
In the oil and gas industry, MEG is widely used in wellheads and pipelines as a hydrate suppressor to prevent hydrate formation at pipeline conditions. On offshore gas production facilities, where the exposure to lower temperatures in subsea pipelines is significant, MEG is in prevalent use for hydrate inhibition. The lean (dry) MEG is injected in the subsea gas pipeline at or near the wellhead and mixes readily with the produced water. The inhibition process is straightforward, with the MEG decreasing the hydrate formation temperature below the operating temperature and thus preventing hydrate blockage of the pipeline.
The now rich (wet) MEG is then dried so that the MEG can be re-used in hydrate control. However, the lean MEG cannot be recovered by simply distilling the rich MEG and water because the rich MEG is loaded with dissolved salt ions from the produced water, including divalent salts of calcium, magnesium, strontium, and barium. If these salt ions are not removed, they will either precipitate or accumulate in the process equipment, eventually leading to failure of downstream treatment processes.
As an example, if calcium salts are allowed to remain in the MEG feed stream, Ca(MEG)4Cl2 may form in the flash separator. This compound, which melts at approximately 95° C. (203° F.), forms a hard solid on cooling. This solid may clog pumps, interfere with heat transfer, and inhibit salt removal in downstream treatment processes. In addition, plugged equipment must be taken off-line, which reduces the efficiency and increases the cost of the overall treatment process.
Because some salts of divalent ions are highly soluble, they cannot be removed from MEG feed streams by precipitation. Typically, a chemical reaction is employed to alter the species of the divalent ions into an insoluble form which will precipitate. This precipitate can be removed using a variety of techniques. Conventional removal methods include disk stack centrifuges, filter presses, and candle filters. However, each of these methods has disadvantages. Disk stack centrifuges cause the aeration of the centrate, leading to high oxygen absorption. In addition, because the salts cannot be washed, large amounts of MEG are lost as part of the waste slurry. The high MEG content of the waste slurry also complicates disposal by making the slurry difficult to dry. Filter presses are heavy and require relatively large amounts of space, making them generally unsuitable for offshore applications. Candle filters require chemicals, such as pre-coat or body-aid, and large volumes of gas to dry the filter cake, which add capital and operating costs to their use.
A need exists for systems and processes for removing divalent ions from MEG feed streams in order to improve the efficiency of the MEG reclamation or MEG regeneration process and to prevent the accumulation of salts inside the process equipment. A need also exists for systems and processes that are less expensive, require less space, minimize the use of additional chemicals, reduce the frequency of MEG blowdown, decrease MEG loss by recycling it back to the reclamation or regeneration process, and facilitate the disposal of the waste either as a slurry or as solid waste by means of drying. A need also exists for systems and processes that can be located on the main rich MEG feed stream to the MEG processing plant or on MEG feed streams within the MEG reclamation system (e.g., on a side stream off the flash separator vessel or the recycle loop).